1. Field of the Invention
The present invention generally relates to a reactor system for olefin polymerization, and more particularly to a reactor system for optimizing the production of polyolefin polymers in a high-efficiency loop reactor.
2. Related Art
This section introduces information from the art that may be related to or provide context for some aspects of the techniques described herein, claimed below, or both. This information is background facilitating a better understanding of that which is disclosed herein. This is a discussion of “related” art. That such art is related in no way implies that it is also “prior” art. The related art may or may not be prior art. The discussion is to be read in this light, and not as admissions of prior art.
As chemical and petrochemical technologies have advanced, the products of these technologies have become increasingly prevalent in society. In particular, as techniques for bonding simple molecular building blocks into longer chains (or polymers) have advanced, the polymer products, typically in the form of various plastics, have been increasingly incorporated into everyday items. Polyolefin polymers such as polyethylene, polypropylene, and their copolymers, are used for piping, retail and pharmaceutical packaging, food and beverage packaging, plastic bags, toys, carpeting, various industrial products, automobile components, appliances and other household items, and so forth.
One benefit of producing polyolefin is that it is generally non-reactive with goods or products with which it is in contact. This allows polyolefin products to be used in residential, commercial, and industrial contexts, including food and beverage storage and transportation, consumer electronics, agriculture, shipping, and vehicular construction. The wide variety of residential, commercial and industrial uses for polyolefins has translated into a substantial demand for raw polyolefin which can be extruded, injected, blown or otherwise formed into a final consumable product or component.
To satisfy this demand, various processes exist by which olefins may be polymerized to form polyolefins. These processes may be performed at or near petrochemical facilities, which provide ready access to the short-chain olefin molecules (monomers and co-monomers), such as ethylene, propylene, butene, pentene, hexene, octene, decene, and other building blocks of the much longer polyolefin polymers. These monomers and co-monomers may be polymerized in processes comprising a liquid-phase polymerization reactor, gas-phase polymerization reactor, or combinations thereof. As polymer chains develop during polymerization in the reactor, solid particles known as “fluff”, “flake” or “powder” are produced in the reactor.
It was soon discovered that a more efficient way to produce such solid particles of polymers was to carry out the polymerization process under continuous slurry conditions in a pipe loop reactor with the product being taken off from a number of settling legs attached to the bottom horizontal portions of the pipe loop reactor. Multiple settling legs are installed and operated on a batch principle to recover solid polymer products. This technique has enjoyed international success with billions of pounds of ethylene polymers being so produced annually. With this success has come the desirability of building large reactors as opposed to a large number of small reactors for a given plant capacity.
However, the use of multiple settling legs presents at least two problems. First, it imposes a “batch” product recovery technique onto a basic process that requires continuous circulation of a slurry within a loop reaction zone. Thus, problems arise when a settling leg reaches a pre-determined stage where it “dumps” or “discharges” accumulated polymer slurry. Each time a single settling leg operates to recover slurry from the loop reactor, it would cause an interference with the flow of the slurry circulating upstream within the loop reactor and the operation of a reactant recovery system connected downstream. Also, valve mechanisms connecting the settling legs to the loop reactor upstream and the reactant recovery system downstream have to be large in their diameters, thus requiring frequent, seal-off of these valves. There is significant difficulty in maintaining a tight seal. Settling legs may also plug from time to time. Thus, frequent reactor down time for scheduled maintenance is unavoidable, thereby reducing production efficiency and incurring high production cost.
Secondly, as loop reactors have gotten larger, the use of multiple settling legs have inevitably presented many logistic problems. In a loop reactor, if a pipe diameter is doubled, the volume of the loop reactor goes up four-fold. However, because of the valve mechanisms involved, the size of the settling legs cannot easily be increased further. Hence, a larger number of settling legs become necessary to meet the large footprint configuration, which in turn begins to exceed the physical space of the land available for setting up these large-scale, high production capacity loop reactors.
In spite of these limitations, settling legs have continued to be employed where olefin polymers are formed as slurry in a liquid diluent. This is because, unlike bulk slurry polymerizations (i.e. no inert diluent) where solids concentrations of better than 60 percent are routinely obtained, olefin polymer slurries in a diluent are generally limited to less than 50 percent solids. Hence settling legs have been believed to be necessary to give a final slurry product having a greater solids concentration. This is because, as the name implies, settling occurs in the legs, and thus the concentration of the solid particles finally recovered from the slurry settled within the settling legs are higher. Another factor affecting maximum solid concentration within these loop reactors is circulation velocity, with a higher velocity for a given reactor diameter allowing for higher concentration of the solids produced, since a limiting factor in a circulating loop operation is fouling due to polymer build up in the walls of the loop reactor. Without meaning to be bound by any particular theory, such fouling may be due to operation at solid particle concentrations above a level where the solids can remain suspended without settling within the reactor (i.e., saltation).
One way to improve production efficiency compared to the use of multiple settling legs is to employ a continuous product take-off line or other similar mechanism to continuously withdraw slurry products from the loop reactor. In addition, there is a need to reduce the footprint of a loop reactor that takes up less land space (at least in the horizontal dimension) and saves production cost, while still maintaining high production capacity and efficiency. For example, a single continuous product take-off line can replace multiple settling legs, resulting in similar productivity while reducing reactor footprint and simplifying reactor control mechanisms.